In the lithography art, a highlight is recently put on the organic solvent development again. It would be desirable if a very fine hole pattern, which is not achievable with the positive tone, is resolvable through negative tone exposure. To this end, a positive resist composition featuring a high resolution is subjected to organic solvent development to form a negative pattern. An attempt to double a resolution by combining two developments, alkaline development and organic solvent development is under study.
As the ArF resist composition for negative tone development with organic solvent, positive ArF resist compositions of the prior art design may be used. Such pattern forming processes are described in Patent Documents 1 to 3. These patent documents disclose resist compositions for organic solvent development comprising a copolymer of hydroxyadamantane methacrylate, a copolymer of norbornane lactone methacrylate, and a copolymer of methacrylate having acidic groups including carboxyl, sulfo, phenol and thiol groups substituted with two or more acid labile groups, and pattern forming processes using the same.
Further, Patent Document 4 discloses a process for forming a pattern through organic solvent development in which a protective film is applied onto a resist film. Patent Document 5 discloses a topcoatless process for forming a pattern through organic solvent development in which an additive is added to a resist composition so that the additive may segregate at the resist film surface after spin coating to provide the surface with improved water repellency.
The negative tone pattern forming process is also useful in enhancing the resolution of trench patterns. Since a high optical contrast is available where the pattern design on the mask is isolated lines (i.e., bright-field mask), as compared with where the pattern design on the mask is isolated trenches (i.e., dark-field mask), the negative tone patterning process involving reversal of line image is more advantageous in forming trench patterns. See Non-Patent Document 1.
However, it is hardly described that the negative development with organic solvent takes full advantage of optical images even in formation of hole or trench patterns, because of the shortage of dissolution contrast of the above-mentioned resist compositions.
In the negative tone pattern forming process, the contrast shortage tends to result in an inversely tapered profile. There is a likelihood that the line width in a surface layer of resist film is greater than the line width in a substrate-adjoining layer of resist film. Then pattern collapse becomes a serious problem.
In general, the organic solvent development is low in dissolution contrast, as compared with the positive resist system adapted to be dissolved in alkaline developer when deprotection reaction takes place to produce acidic carboxyl or phenol groups. Specifically, in the case of alkaline developer, the alkali dissolution rate differs more than 1,000 times between unexposed and exposed regions, whereas the difference in the case of organic solvent development is at most 100 times, and only about 10 times for certain materials. Further it is important to enhance the gradient or gamma (γ) at the dose corresponding to dissolution/non-dissolution conversion. A low γ value is prone to form an inversely tapered profile and allows for pattern collapse in the case of a line pattern. To obtain a perpendicular pattern, the resist must have a dissolution contrast having a γ value as high as possible.
It is also important to control acid diffusion in a resist film because the diffusion of acid generated in the exposed region of resist film to the unexposed region invites degradation of a latent image and the degraded profile leads to a loss of pattern collapse resistance and an increase of edge roughness.